Diagnostic medical ultrasound scanners have become indispensable tools for clinicians because they can provide blood-velocity information in real time. Spectral Doppler (blood-velocity spectra displayed as a function of time) provides quantitative information from a single range-gated area that is vital in such areas as vascular disease, cardiac function and fetal health, while color flow mapping (superposition of color coded mean velocities and variances on a grey-scale anatomical image) compliments this and is more commonly used for qualitative analysis of blood flow.
While the routine use of these tools is rapidly increasing, they can detect and display only a one-dimensional projection (onto the axis of the ultrasound beam) of the natural three-dimensional blood flow velocities. In spectral Doppler, this projection is calculated using an angle correction that can be estimated by using the surrounding anatomy, such as the orientation of vessel walls, as a guide. However, this manual correction is rarely accurate or repeatable and flow may not necessarily be parallel to vessel walls. In color flow mapping, mean velocities are calculated with no angle correction, resulting in an angle dependency that may vary across the image. This dependency is influenced both by the scanning geometry of the transducer and the anatomy that is being scanned (e.g., curvature of vessels). The angle dependency of current systems and the chosen display methodologies often produce inhomogeneous images that are not good representations of the actual flow field. More recently, many of the ultrasonic imaging apparatus manufacturers have introduced a new feature in their machines called "power-mode Doppler", which acts to color code the estimates of the instantaneous power rather than instantaneous mean frequency, of the received Doppler signal. While this offers an improvement in sensitivity and eliminates some of the inhomogeneities of normal color flow mapping, vital velocity and directional information is rejected in the process.
To overcome these limitations, scanners must detect and display more than a one-dimensional projection of the three-dimensional blood flow velocity vectors. Prototypes have shown that this can be done but none has been introduced commercially or used on a routine clinical basis due to a number of common limitations. These limitations independent or in combination are among the following:
(1) limited fields of view: many attempts have only found success using a single point or range gate within a B-mode image; PA0 (2) Impractical transducer assemblies: often these assemblies consisted of more than one transducer fixed into a large impractical handle that was not useful in the clinical environment; PA0 (3) Too few piezoelectric elements and many attempts did not utilize a phased array transducer but rather a few single-element piston transducers; PA0 (4) Alignment problems: mis-aligned interrogating beams were not from the same point in space; (5) Reduced frame-rates: real-time operation was not possible with the method chosen; and PA0 (6) Non-simultaneous interrogations from more than one angle: for example--techniques that relied on ECG triggering to acquire multiple interrogations lost the attraction of being real-time and were often susceptible to inaccuracies due to acceleration of the blood flow in between interrogations.
One embodiment of the novel apparatus of the invention measures the magnitude and direction of blood flow velocity vectors over an adequate two-dimensional field of view at frame rates equivalent to those presently available in commercial scanners. The technique uses conventional phased array transducers or more specialized phased array transducers and demands only a modest increase in processing power. Following the theoretical description hereinbelow, applicants present an error analysis based on a set of simulations which demonstrate the apparatus' ability in conveying unambiguous and accurate velocity estimates. Then applicants present results from experimental work which used a blood-mimicking phantom, further demonstrating the feasibility of the invention for the case of measuring two-dimensional velocity vectors. This is followed by a discussion of results and a practical description of the invention including a description of the optional novel quadrature circuitry used in the practice of the invention.